The maize mycorrhizosphere as a source for isolation of arbuscular mycorrhizae-compatible phosphate rock-solubilizing bacteria

Aims Phosphate chemical fertilizers are costly and raise concerns about environmental pollution through industrial production. The use of phosphate rock (PRs) emerges as a more sustainable alternative for agriculture. The aim of this work was to isolate phosphate rock-solubilizing bacteria (PRSB) from maize mycorrhizosphere, having growth promoting traits and that will be arbuscular mycorrhizae fungi (AMF) compatible. Methods Bacteria were isolated from the mycorrhizosphere of maize and tested for rock phosphate solubilization, production of organic acids, phosphatases and phytase activities, and growth promotion traits. The capacity of some strains to enhance the dry weight of maize plants was determined in a growth chamber experiment. The compatibility of the selected strains withRhizophagus irregularisunder in vitro conditions was also tested. Results Out of 118 isolates from maize, eight belonging toAsaia lannaensis,Rahnellasp.,Pantoeasp., andPseudomonassp.were found to be the best PRSB. On solid media, all strains mobilized P from tricalcium phosphate, hydroxyapatite, and PRs.A. lannaensiswas the only PRSB showing visible solubilization of AlPO(4)and FePO4. All the PRSBs solubilized PR by producing D-gluconic acid and 2-ketogluconic acid and by lowering the pH. Most strains presented IAA and siderophore production and different biofilm formation and motility capacities. All strains improved the dry weight of maize seedlings compared with non-inoculated plants. The results proved that PRSBs were able to grow onR. irregularishyphae as the sole in vitro C source. Conclusions Bacteria isolated from the mycorrhizosphere of maize shows effective solubilization of phosphorus from PR with different reactivity levels. The traits of these bacteria as growth promoters and their biocompatibility with AMF show their potential as inoculants. Improvement of the agronomic effectiveness of PRs is relevant for developing countries that use PRs directly as P-fertilizers (less expensive than soluble P-fertilizers) for sustainable agriculture.